In fact, researchers at the Yale School of Medicine observed that exposing human immune cells to BHB in amounts you'd expect to find in the body following 2 days of fasting resulted in a reduced inflammatory response (2).

Furthermore, recent research on mice and humans showed that fasting for 48–72 hours may also promote the recycling of damaged immune cells, allowing for the regeneration of healthy ones (3).

It's important to mention that the exact ways in which fasting affects the immune system are not yet fully understood. More studies are needed.

Bottom Line: Short periods of fasting may support healthy immune function by promoting immune cell recycling and limiting the inflammatory response.

Common cold and flu-like symptoms can be caused by either viruses or bacteria.

To be perfectly clear, cold and flu infections are initially caused by viruses, specifically the rhinovirus and influenza virus.

However, being infected with these viruses lowers your defense against bacteria, raising your chances of simultaneously developing a bacterial infection, whose symptoms are often similar to your initial ones.

Interestingly, there is research to support the idea that the lack of appetite you often feel during the first few days of an illness is your body's natural adaptation to fighting the infection (4).

Below are three hypotheses that attempt to explain why this might be true.

From an evolutionary perspective, lack of hunger eliminates the need to find food. This saves energy, reduces heat loss and essentially allows the body to focus solely on fighting off the infection (5).

Abstaining from eating limits the supply of nutrients, such as iron and zinc, that the infecting agent needs to grow and spread (6).

The lack of appetite often accompanying an infection is a way to encourage your body to remove infected cells through a process known as cell apoptosis (7).

Interestingly, results from a small study suggest that the type of infection may dictate whether eating is beneficial or not (8).

This study suggested that fasting may best promote healing from bacterial infections, while eating food may be a better way to fight viral infections (8).

A previous experiment in mice with bacterial infections supports this. Mice that were force-fed were less likely to survive compared to mice allowed to eat according to appetite (9).

All the studies so far seem to agree that the beneficial effects of fasting are limited to the acute phase of infection — usually lasting up to just a few days.

However, there are currently no human studies examining whether fasting or eating have any effects on the common cold or flu in the real-world.

Bottom Line: Many hypotheses attempt to explain how fasting can help promote healing, but more research is needed to confirm the effects in humans.